Background: According to some current guidelines, extended thromboprophylaxis after hip and knee arthroplasties is recommended. Outpatient prophylaxis with low molecular weight heparins (LMWH) is an important part of this prophylaxis, although the rates of adherence to these regimens is not known. Questions/purposes: We determined (1) the degree of nonadherence (NA) of patients with LMWH outpatient prophylaxis, and (2) whether specific independent factors explain NA. Methods: NA was determined by syringe count and by indirect and direct questions to patients. We defined six different NA indicators. To identify factors explaining LMWH NA, we used three different logistic regression models. Results: NA rates ranged between 13% and 21% depending on the indicator used for measurement. Patients who were nonadherent missed between 38% and 53% of their outpatient LMWH injections. If patients attended an outpatient rehabilitation program, the probability for their NA increased substantially. Moreover, the NA probability increased with each additional day between acute hospitalization and start of rehabilitation (linking days). NA was lower for patients who feared thrombosis or who believed antithrombotic drugs to be the most important measure in thromboprophylaxis. Level of Evidence: Level II, prognostic study. See the Guidelines for Authors for a complete description of levels of evidence.